It is well known that many desirable characteristics of bitumen can be improved by combining with it certain polymeric materials, especially elastomeric materials. For example, European Patent Publication No. 317,025 to Shell International Research disclosed a bitumen composition useful in road paving applications containing an asymmetric radial block copolymer which exhibits increased toughness and tenacity. PCT Publication No. WO 90/02776 to Elf Aquitaine, disclosed a rubberized bituminous composition which was modified through in-situ vulcanization of a copolymer of styrene and a conjugated diene with a coupling agent, such as sulfur.
The incorporation of crumb rubber from recycled automobile and other tires into bitumen or asphalt is desirable in view of the potentially improved properties of composition attained thereby and the recycle of scrap rubber achieved thereby.
Scrap crumb rubber represents a significant source of rubber vulcanates, which contain a variety of rubber polymers, predominantly styrene-butadiene rubber. Crumb rubber generally is recycled rubber that has been reduced to ground or particulate form by mechanical shearing or grinding. It has been proposed that scrap crumb rubber be incorporated into asphalt paving materials. In general, crumb rubber is blended into asphalt paving materials by one of two processes, namely a wet process or a dry process.
In the dry process, the rubber crumb is added to the heated aggregate, not the asphalt cement, or to the hot mix asphalt mixture during production of the mix. In such dry mix processes, beneficial chemical changes to the asphalt binder, such as bond-cleavage or stabilization of additives, are extremely unlikely.
In wet processes, on the other hand, beneficial changes to the properties of the binder, such as those disclosed in the present invention, can be readily achieved by the appropriate blending of additives, usually polymers. In practice, the crumb rubber is blended into the asphalt cement, by batch blending in which batches of crumb rubber and asphalt are mixed in production, by continuous blending with a continuous production system, or by terminal blending. An asphalt cement binder that has been modified with crumb rubber is termed asphalt rubber.
In one wet procedure in which polymers are used, hot asphalt (about 190.degree. to 220.degree. C.) is mixed with approximately 25 to 30 wt % crumb rubber and the mixture then is diluted with kerosene. A variation of this procedure uses about 22 wt % crumb rubber with dilution being effected using extender oil. It is thought that blending the crumb rubber and asphalt at elevated temperature may promote limited chemical bonding of the components. However, these compositions exhibit only short-term stability and, therefore, must be employed shortly after formation.
A recent variation of the wet process is described in U.S. Pat. No. 4,992,492. The process involves a mixture of asphalt or sulfur-treated asphalt (81 to 86%), crumb rubber (8 to 10%), extender oil (4 to 6%) and a high molecular weight (&gt;100,000) olefinically-unsaturated synthetic rubber (2 to 3%) which is blended together at 175.degree. to 180.degree. C. for about two hours.
As claimed, this process differs from the present invention in a number of important facets. In the referenced process, the ground crumb rubber is dispersed in the bitumen, however, the vulcanizate network undergoes limited, if any, chemical disassociation. Such crumb rubber compositions would be unstable without the incorporation of the claimed high MW (.gtoreq.100,00) olefinically-unsaturated synthetic rubber. The high MW free solvated synthetic rubber chains likely act to minimize changes in viscosity and softening point over periods of up to 10 days in a "hermetically-sealed vessel without agitation at 160.degree. C. to 165.degree. C." to promote stability. Other variations of the wet process are described in WO 95/20623 and EP 439,232.
In WO 93/17076, ground rubber particles are heavily oxidized, particularly at the surface of the particles, with air injected under pressure at a high temperature (220.degree. C. to 260.degree. C.), in a procedure similar to that employed conventionally for producing an oxidized or "blown" asphalt for roofing-grade asphalt. Such treatment of the fine rubber particles in situ improves desired rubber dispersibility and compatibility, but also may imparts undesirable brittleness to the asphalt matrix.